This invention relates to a method and apparatus for eliminating or at least drastically reducing the cracking which today frequently occurs at the junctions of the body and shank of ferrous alloy die blocks and similar parts.
Die blocks are well known forging implements which, after the sinking of an impression therein to thereby form a die, are used in forging machines such as hammers. A hammer die, after final machining and heat treatment, is then fitted to a die holder in the hammer. A typical hammer die has a large thick body (to provide for one or more resinkings of the impression) and, usually, a relatively short, dovetailed shaped shank located in the middle of one side of the body and extending the length of the body. A typical shank is about 2xe2x80x3 in height.
In operation a hammer die is exposed to extremely rugged conditions. In normal operations with all machine components properly positioned and secured, tremendous shock loads are transmitted to all portions of the die. Such loads, which are derived from the many tons of impact forces resulting from the weight of the downwardly driven ram portion of the hammer die striking the workpiece resting in the die holder of the hammer die, have their greatest effect on the weakest portion of the die which, as is well known, is the junction of the shank and body of the hammer die. All too frequently the dies, which may range in hardness from about 28Rc to about 54Rc, are cracked or fractured at the shank-body junction of the die and this can lead to catastrophic failure.
Many forging die applications require a tool steel die block that has been heat treated to a high hardness level to optimize the wear resistance of the working face. At the same time the shank portion of the die block requires a lower hardness level to facilitate machining and prevent cracking of the filet radius during the forging process. The xe2x80x9ccompositexe2x80x9d design is achieved by heat treating the entire block to the high face hardness and then selectively tempering the shank portion at a tempering temperature higher than that used to temper the entire block.
In the current practice the shank is tempered by immersing a portion of the previously heat treated and hardened die block into a bath of molten metal salt containing barium chloride (BaCl2) at a temperature of 1250xc2x0 F. (677xc2x0 C.). Heat from the molten salt is conducted into the submerged portion of the die block, is transmitted through the block, and is lost through radiation and convection from the portion of the block exposed to the ambient air above the salt. After approximately 180 minutes a steady state heat transfer condition is established where the highest temperature of approximately 1250xc2x0 F. (677xc2x0 C.) is present at the submerged corner. The temperature decreases to approximately 1050xc2x0 F. (566xc2x0 C.) at the salt immersion depth. The temperature continues to decrease toward the top surface of the die block exposed to the ambient air. The final temperature at the top (working face) of the die block depends on the depth immersion and total height of the die block. It is imperative that the working portion of the die block remain below the original die block tempering temperature to prevent softening of the working face. The metallurgical effectiveness of the shank tempering process depends on the combination of the temperature achieved and time held at that temperature. The current practice specifies a total salt bath treatment of 6 hours (3 hours after steady-state is reached) to allow for sufficient tempering of the shank portion.
Technical, maintenance, environmental, and safety problems limit the commercial success of the current process. Technically the process is limited by the relatively slow rate of heat input generated by the molten salt at 1250xc2x0 (677xc2x0 C.). The slow heat input rate coupled with the heat lost due to radiation and convection from the portion of the block exposed to the ambient air limits the maximum temperature within the block, at that salt immersion depth, to approximately 1050xc2x0 F. (566xc2x0 C.). The extent to which the shank is selectively tempered is limited by the temperature achieved in the shank portion of the die block and the time held at temperature. The maximum temperature of the top (working face) must remain below the original tempering temperature of the parent block to prevent softening. This maximum working face temperature depends on the depth of immersion into the salt bath (heat input) and the height of the block above the salt bath (heat output). For small blocks it is impossible to sufficiently temper the shank portion without softening the working face due to the relatively small portion of the block above the salt bath. Further the process is somewhat time consuming requiring a batch processing time of six hours. It is possible to increase the effective tempering temperature at the salt immersion depth and decrease the batch processing time by increasing the temperature of the molten salt bath, however, this only increases the maintenance, environmental, and safety problems associated with the process.
Several maintenance problems hinder the commercial success of the salt bath shank tempering process. Costly stainless steel pots are used to contain the molten salt used for the shank tempering process. These pots are corroded by the salt and require replacement approximately every eight months resulting in an annual cost of $5,700. Any increase in salt pot operating temperature will significantly reduce the life of the salt pots. The actual metal salt must be replenished at a cost of approximately $2,000 annually. In addition to the cost of these consumables is the annual cost of approximately $21,000 for the natural gas used to heat the pot. Additional costs are associated with the maintenance of the burners, themocouples, and the control systems.
Several environmental and safety problems plague the use of the salt bath shank tempering process. The barium chloride contained in the salt is considered a hazardous waste under the Resource Conservation and Recovery Act due to its barium content which is a heavy metal and requires special disposal procedures. Overexposure to this salt can lead to several varied health risks. Skilled operators are required to conduct the salt bath processing due to the many safety hazard associated with the molten salt. Extreme care must be taken to avoid the introduction of water into the molten salt. Condensation or ice that may have accumulated on the die blocks will become explosive upon contact with the molten salt if not thoroughly removed prior to immersion in the bath. If moisture is introduced the rapid conversion to steam can splatter the molten salt onto adjacent personnel. Care must also be taken when placing blocks into the salt bath to avoid inhalation of the powdered metal salt when loading the pot. Because of these environmental and safety concerns it is required that any salt bath tempering process must be located in a specialized shop area.
Following the salt bath treatment the blocks must be stored until cool. Next, the salt that adheres to the sides of the block must be removed prior to the moving the blocks to the next operation. Again this is required to contain the metal salt and prevent contamination of other locations. The same precautions must be maintained when handling the salt that is removed from the sides of the block.
The results of such treatment, while better than no treatment, are, in a sense, marginal since the process is difficult to regulate and measure with precision and a substantial element of judgment enters into the practice of the process, even on a day-in-day-out routine basis. Further, the process is lengthy, often requires the use of cranes or other auxiliary equipment to manipulate, hold and control the position of the die block during the salt bath treatment. The blocks, which are custom made, are of different sizes, shapes and widths, and this non-uniformity makes it even more difficult to properly reduce the hardness at the inside corner of the shank cut-out.
In summary the operating drawbacks to the salt bath system may be summarized as follows:
1. Salt pot has to be replaced twice a year at a cost of approximately $4,000.
2. The salt bath is a toxic waste and disposal is difficult.
3. Salt pot is labor intensive.
4. Salt pot has to be in a special, protected location.
5. Splash and inhalation from the salt is dangerous to the operator.
6. Periodic cleaning is necessary.
7. Salt sticks to sidesxe2x80x94has to be washed off.
8. There is a danger of explosion due to the presence of water or ice on the die block.
There is therefore a need for a method and apparatus for preventing, or at least reducing the incidence of; cracking at the shank-body junction of die blocks which is speedy in application, requires minimal handling of the die block to be treated, minimal auxiliary equipment during processing, eliminates the use of hot, liquid salt baths with their above described drawbacks, and gives predictable and duplicatable results over the range of sizes, shapes, and compositions of die blocks currently produced.
The invention is a shank-body drawing or tempering system utilizing electric heat that eliminates the need for the currently used salt baths with their attendant drawbacks as described above, yet which can process all shapes, sizes and compositions of die blocks in a speedy, efficient and reproducible manner with consistent results, while requiring only a fraction of the cost of capital equipment and operating costs of salt baths, including savings in manpower, space and consumable materials.
In a first embodiment of the invention paddle shaped induction heater means are placed in operative contact with a ferrous workpiece and an enclosure which does not transmit induction currents, said paddle including induction heating coil means having a capacity to heat the critical areas of the die block to any desired depth and any degree of softness using well known operating parameters currently utilized in induction heating devices. Preferably a die block is placed, in a shank down position, on a non-magnetic base and an induction heating paddle is placed in contact with the shank, the exposed portion of the paddle being blocked off with non-magnetic material. The water cooled copper tube induction coil, which is encased in a non-magnetic jacket, is activated for a sufficient period of time, depending on size, shape and composition of the workpiece, to temper the shank-body junction to a condition in which cracking is either eliminated or drastically reduced as contrasted to the results currently achieved with salt baths or other means.
In another embodiment of the invention the die block after hardening but either before or after a shank is formed in the back side (i.e.: the non-working surface) of the die block is subjected to infrared heat. The infrared heat is preferably generated by tungsten halogen lamps which are arranged to direct the radiant energy at the surface to be treated. While no limits on the length of the waves of the electromagnetic spectrum have been positively established, good results have been obtained with short wave radiation, i.e.: 0.78 to 2.0 xcexcm.